1. Field of Invention
The present invention relates to a liquid crystal alignment film and a process for its production, as well as an optical element using the liquid crystal alignment film and an optical information writing/reading device using the optical element. More particularly, it relates to a liquid crystal alignment film useful for preparing, by means of optical anisotropy of liquid crystal, an optical element to be used for optical pickup, imaging or communication, and a process for its production, as well as an optical element and an optical information writing/reading device using the optical element.
2. Discussion of Background
At the time of reading out information recorded on an optical disk or writing information on an optical disk, an optical element for shifting (polarizing, diffracting, phase-adjusting, etc.) laser beam is required. For example, at the time of reading out information, linearly polarized light emitted from a laser light source reaches an optical disk surface via a polarizing element and then via a retardation plate. The outgoing linearly polarized light will pass linearly through the polarizing plate since it is aligned in a direction not to be changed by the polarizing plate and will then be shifted to circularly polarized light by the retardation plate. This circularly polarized light will be reflected by the information-recording surface of the optical disk to form a reversely rotated circularly polarized light, and will then be converted again by the retardation plate to linearly polarized light, of which the polarization direction is perpendicular to one prior to incidence. This returning light flux will have its travelling direction bent at the time of passing again through the polarizing element and will then reach a light receiving element.
Further, at the time of reading out or writing information, if face deflection of an optical disk or the like occurs, the focus position of the beam spot is displaced from the information-recording surface, and therefore, a servo mechanism is required to detect and correct the focus position to let the beam spot follow the concavoconvex pits on the information-recording surface. Such a servo system for an optical disk is constructed to detect the position of the truck after adjusting the focus of the beam spot radiated from a laser light source on the information-recording surface and to let the focus position follow the desired truck. Further, it is also required to make sure that laser beam reflected without hitting pits on the information-recording surface will not be returned, as it is, to the light source.
For this purpose, an optical element to shift the laser beam is required for an optical information writing/reading device. For example, a retardation plate (a wavelength plate) has an effect to give a different refractive index to incident light, by an angle between the optical axis of the retardation plate and the phase plane of incident light and further to displace the phases of two component lights formed by birefringence. The two lights having the phases displaced, will be combined when emitted from the retardation plate. The displacement of the phases is determined by the thickness of the retardation plate, and therefore, by adjusting the thickness, it is possible to prepare a quarter wavelength plate to displace the phases by π/2, a half wavelength plate to displace the phases by π, etc. For example, linearly polarized light passed through a quarter wavelength plate becomes circularly polarized light, and linearly polarized light passed through a half wavelength plate becomes linearly polarized light having its polarization plane tilted by 90°. By utilizing such natures, a combination of elements is applied to e.g. a servo mechanism. Such an optical element is useful for not only an optical pickup element utilized to read out information from an optical disk, but also an imaging element for projectors, etc., a communication device for tunable filters, etc.
Further, such an optical element may be prepared from a liquid crystal material. Liquid crystal molecules having polymerizable functional groups have both a nature as a polymerizable monomer and a nature as liquid crystal. Therefore, when liquid crystal molecules having polymerizable functional groups are aligned and then polymerized, it is possible to obtain an optical anisotropic material having alignment of liquid crystal molecules fixed. The optical anisotropic material has an optical anisotropy such as a refractive index anisotropy derived from a mesogenic structure, and is used for a diffraction element, a retardation plate, etc. by utilizing such a nature.
Further, for multifunctionality of an optical element, multidomain alignment is desired. Once a multidomain alignment pattern can be formed, it is possible to change the outgoing polarized light pattern for every domain. For example, by controlling the outgoing polarized light pattern, it is possible to prepare a depolarization element, a polarized light transmittance-controlling element or a diffraction element. The multidomain alignment is meant for an alignment pattern having differently-aligned domains in one substrate. The multidomain alignment may be formed by a process such as a photo-alignment method, a mask rubbing method or a groove alignment method. However, in consideration of the productivity, costs, alignment to liquid crystal or refinement of patterns, photo-alignment is preferred. In the marketplace, the multidomain alignment pattern is used as a new technique for a 3D retardation film or a liquid crystal panel.
In a photo-alignment process, a photo-alignment film formed on a substrate is irradiated with polarized ultraviolet light, whereby the alignment direction of liquid crystals is controlled by the light. In such a photo-alignment process, the alignment-regulating power may be provided only at a domain irradiated by the light, and by using a mask to partially shield the light, such as a Cr mask, it is possible to easily form domains having different alignment directions in the same substrate plane.
In the alignment technique using the photo-alignment, it is possible to align liquid crystals by irradiation with polarized ultraviolet light without requiring conventional rubbing treatment, and by this method, there will be no formation of static electricity or scratches on the film surface. As a photo-alignment material which has been reported, one using a dichromatic azo dye may be mentioned (Patent Documents 1 to 3). According to this method, a film surface is irradiated with polarized ultraviolet light, and by utilizing photo-isomerization of the azo dye, liquid crystal molecules are aligned in a certain direction.
As another system, a system has been studied wherein an organic film containing a cinnamate structure (Patent Documents 4 to 6) or a chalcone structure
(Patent Documents 7 to 10) is irradiated with polarized ultraviolet light to form a cross-linked structure thereby to provide an alignment-regulating power.
Further, a system has also been reported wherein a polyimide film is irradiated with polarized ultraviolet light to cause an anisotropic photodecomposition thereby to impart anisotropy to the polyimide film and thus provide an alignment-regulating power (Patent Document 11).
A problem common to the above-mentioned conventional photo-alignment films is such that the light resistance against blue laser is inadequate. This is attributable to conventional photo-alignment materials having such a structure that in order to increase the sensitivity to polarized ultraviolet light, their absorption of ultraviolet light is made high. If the conventional photo-alignment films are exposed to blue laser beam (405 nm) for Blu-ray applications, the alignment films themselves undergo deterioration caused by absorption, and such conventional materials are not able to satisfy the level of light resistance in Blu-ray applications which increases year after year. Not only in Blu-ray applications, but also with respect to optical elements in imaging applications, severe light resistance is required against green laser of 532 nm, and the light resistance of the alignment films themselves becomes very important, since it is directly influential over the light resistance of the entire optical elements.
The light resistance of a film usually tends to be improved as the film becomes thin. A method has been proposed wherein a single layer of a silane coupling agent is used to have temporary alignment formed in a drain liquid off direction by an organic solvent, thereby to increase the sensitivity for photocrosslinking during irradiation with polarized ultraviolet light, and thus is used as a liquid crystal photo-alignment film (Patent Documents 10, 12 and 13).
Patent Document 1: JP-A-2006-285197
Patent Document 2: JP-A-5-232473
Patent Document 3: JP-A-2002-250924
Patent Document 4: JP-A-11-189665
Patent Document 5: JP-A-6-289374
Patent Document 6: JP-A-8-15681
Patent Document 7: JP-A-11-152475
Patent Document 8: JP-A-10-251646
Patent Document 9: JP-A-2002-258303
Patent Document 10: JP-A-2001-100214
Patent Document 11: JP-A-9-5747
Patent Document 12: JP-A-11-258611
Patent Document 13: JP-A-11-125821
However, in a case where a single layer of a silane coupling agent is used as disclosed in Patent Documents 10, 12 and 13, the alignment-regulating power changes depending upon the temporary alignment direction, and in order to align a chemisorbed film, a compound having a cinnamate or chalcone structure is used, whereby the light resistance tends to be inadequate.
Further, in addition to the above-mentioned problem relating to light resistance, conventional photo-alignment films have had a problem such that particularly in a case where liquid crystals having two or more polymerizable groups are aligned and polymerized, adhesion between the liquid crystal interface and the alignment film tends to be poor, thus leading to a problem in reliability or peeling due to deficient adhesion.